In February 2007, a naked, emaciated, mutilated, charred and stabbed man is discovered near railway tracks in the Parisian suburb of Sainte Geneviève-des-Bois. He is taken to hospital where he is pronounced dead just before noon. Two days later, the victim is identified as Ilan Halimi, a 23-year-old Jew who was abducted while working in a cell phone shop. He was held hostage and tortured for three weeks by a group calling itself the Gang of Barbarians in a housing estate in Bagneux, a suburb south of Paris. Within days, dozens of arrests are made. Gang leader Youssouf Fofana, who had fled to Ivory Coast, is quickly extradited and imprisoned.

The kidnap, torture and murder of Ilan Halimi vividly illustrates French society's ills in the first decade of the 21st century. The unrepentant gang leader, Fofana, who called himself (in English) the "Brain of the Barbarians", is the French-born son of immigrants from the Ivory Coast. He is a small-time thug driven by Islamic Jew-hatred. Asocial and amoral, tyrannical and seductive, cruel and clumsy, he thrives on delusions of grandeur drawn from the jihadist playbook. Having botched dozens of other attempts at extortion, he finally succeeded in committing an atrocious murder. Since 27 April, his case has been heard behind closed doors.

The 27 defendants, accused of direct or indirect involvement in Halimi's kidnap and torture, are not all Muslim. But they all allegedly participated in a crime inspired by Islamist anti-Semitism. The police were clearly determined to return Ilan to his family safe and sound. However, they worked with an outdated protocol for dealing with ransom demands, refused to accept that the gang had anti-Semitic motives, never understood their psychology and as a result failed miserably.

It was virtually impossible to verify what little information was made available when the crime was discovered, because reporting restrictions were imposed during the long inquest. Nothing filtered out, except for the occasional story of Fofana's outrageous threats against judges, the courts and anyone else who angered him. He accused them all of being Jewish. Disingenuous ambiguity clouded the issues — was it really an anti-Semitic crime? Did it have anything to do with Islam? Today there is barely any coverage of the case because of the reporting restrictions. However, there is a Nouvel Observateur blog, run by Elsa Vigoreux, who publishes information from anonymous sources.

The case is being heard in juvenile court because two of the defendants, including Yalda, an Iranian girl who was sent to lure Ilan, were just under 18 when the crime was committed. They could have waived their rights to a trial in camera. They didn't. They could have saved Ilan's life with an anonymous tip-off to the police. They didn't.

Fofana had sent Yalda and another girl to lure Jewish victims in cell phone shops near the Place de la République. Jews had money, he told Yalda, and they stuck together. If the family can't pay the ransom, the community will pitch in. Jews, he told her, lived like kings in France while we lived in misery. After the crime was uncovered, commentators gave an economic narrative that would hide the truth of murderous Jew-hatred flourishing in a Parisian banlieue. It wasn't really anti-Semitism, they claimed. It was simply that Fofana thought Jews were rich. The police, too, stubbornly clung to the kidnap-for-ransom scenario.

What did they make of the photo the Barbarians sent the family the day after Ilan disappeared? The photo was reproduced on the cover of Choc magazine on 18 May. Withdrawn by a court order, it still circulates on the internet. Ilan's face is completely covered with thick silver duct tape except for his broken, bleeding nose. His hands are bound with the same tape. A newspaper is propped against his chest and he is holding his car keys. ("Key" was the code word Yalda used to signal to the thugs waiting for Ilan behind the bushes.) A black-sleeved hand holds a gun to Ilan's head. Does that look like a kidnapping for ransom? Ilan's eyes and mouth are taped shut. Doesn't that indicate exceptional cruelty and clumsy incompetence? Exaggerated, erratic ransom demands ranged from €5,000 to €450,000 (£4,250-£380,000). Drop-off appointments were made and cancelled. Koranic verses were read against the background of Ilan's screams.

Although Ilan lived with his mother, the police decided that her ex-husband — and his father — should be the kidnappers' sole contact. He took as many as 50 phone calls in one day, all of them peppered with murderous threats and anti-Semitic insults. And yet the police could never trace them.

Ilan was held in a vacant apartment and then transferred to a basement before workmen came to paint the apartment for the new tenants. The duct tape was never removed from his face, his hands were constantly bound, he was naked under a flimsy robe in the dead of winter. He was fed through a straw. His toilet was a plastic bag.

Some of the gang members are charged as accessories, others with direct participation in kidnapping, illegal confinement and torture, with the aggravating circumstances of anti-Semitism. They guarded the prisoner, beat him, burned him, cut out chunks of his skin, taunted him, threatened him, deprived him of basic human needs and watched him creep slowly to inexorable death. Twenty-four days, 576 hours, 34,560 minutes of agony.

On 13 February, having failed to get the ransom money, Fofana stuffed what was left of Ilan into the boot of a stolen car, drove to a field near a railway line in nearby Sainte Geneviève-des-Bois, doused him with flammable liquid, set fire to him and stabbed him in the neck and hip.

No one knows what would have happened if the police had grasped the nature of the criminals. How many hostages have been beheaded by jihadis in recent years? Governments, armies and investigators have been stumped by this new type of violence. Maybe the Barbarians would have murdered Ilan immediately had they sensed the police hot on their trail. Nevertheless, the failure to understand the anti-Semitism behind the crime, and the inexplicable bungling of repeated opportunities to flush out the criminals, locate their hideout, trace communications and connect Ilan's abduction to previous attempts with the same target — Jews — and the same operating method cannot be dismissed.

In a riveting, heart-rending book published in April, 24 jours, la vérité sur la mort d'Ilan Halimi (24 days, The truth about the death of Ilan Halimi), Ilan's mother, Ruth, recounts her ordeal and explores the larger issues with dignity and fairness. Expressing gratitude and respect for the detectives who stood by her, she nevertheless deplores their failure to trust her intuition about the psychology of the Barbarians who transformed her beloved son into a filthy object to be tormented to death. Unable to understand the Jew-hatred spewing from the mouth of Fofana, says Ruth, agents misled Ilan's father in his communications with the kidnapper.

Shortly after the discovery of the atrocious crime, then President Jacques Chirac dispatched his personal counsel, Maître Francis Szpiner, to represent the Halimi family. But Szpiner is not known as a great friend of the Jews, having been part of the defence team of TV station France 2, which lost an appeal against a libel verdict it had won over a report about the killing of a Palestinian boy, Muhammad al-Dura, in 2000. Al-Dura, who was seen cowering behind his father, became the poster boy of the second intifada.

Media critic Philippe Karsenty had been convicted of libel for writing that France 2's report of the death of al-Dura was a hoax. The Paris court ruled that the extensive evidence produced by Karsenty, including a ballistics report and a detailed analysis of the raw footage, was sufficient cause for suspicion that the scene had been staged.

In his aggressive closing argument, playing on the definition of a Zionist, Szpiner compared Karsenty to "a Jew who pays another Jew to send a third Jew to go to war against the Palestinians."

Two members of Fofana's defence team — Maître Isabelle Coutant-Peyre, the wife of Carlos "the Jackal", the Venezuelan-born pro-jihadi serving a life sentence for multiple terror attacks, and Maître Emmanuel Ludot, who represented Saddam Hussein — were interviewed on an Agence France Press video on the opening day of the trial. Angered by hecklers shouting at defendants and lawyers outside the court, they denounced political and media pressure against their client, claiming that President Nicolas Sarkozy was using the case for ignoble electoral reasons.

They also claimed that some of the plaintiffs were backed by a "certain lobby", and that blacks had been attacked by thugs from the right-wing Zionist Betar and the Jewish Defence League groups "when the Halimi family organised a demonstration". Maître Coutant-Peyre declares in the video: "Fofana is a scapegoat." A young lawyer joins them. They discuss the case. He thinks they'll be able to get the court to drop the aggravating circumstances of anti-Semitism. They joke about Maître Szpiner. Is the Elysée (the presidency) paying his fees? The young lawyer guffaws. "It's the Crif [the umbrella body of French Jewry]!" he says, provoking derisive laughter, "and the Elysée is funding the Crif." According to leaked information, Fofana subsequently dismissed Coutant-Peyre in an outburst of paranoid anti-Semitic rage, shouting, "Peyre, that's a Jewish name, isn't it?"

In the absence of reliable information about the trial, which is scheduled to run until 11 July, how can one predict the verdict? The death penalty was abolished in 1981. Life imprisonment is a relative concept. Prisons are overcrowded and dangerous criminals are often released early. Confidential sources have told me that Fofana could be "rubbed out" in prison.

Journalists who were present at the start of the trial, until a motion brought by the Halimi family to hear the case in an open court was defeated, reported that Fofana entered shouting "Allahu Akhbar" ("Allah is Great"). Asked to identify himself, he replied in mangled French, "Arabs, African armed revolt, Salafist barbarian." He gave the day of Ilan's death as his date of birth.

Will lawyers, if any are left to defend him, use Fofana's megalomaniac defiance as an argument for diminished responsibility? In 2003 a Muslim neighbour lured a Jewish DJ, Sebastien Selam, into the underground garage of their building, slit his throat, gouged out his eyes with a carving fork, went home and told his mother: "I killed my Jew, I'll go to paradise." He was released after spending a few years in a mental hospital and will apparently never be tried. The anti-Semitic motivation in that case was so thoroughly denied that commentators systematically referred to Halimi's killing as the first anti-Semitic murder in France.

Whenever immigrant youths from the banlieue are concerned, French authorities walk on eggshells for fear of igniting mass revolt. Which brings us back to Eva Vigoreux's Nouvel Observateur blog. We discern a defence strategy aimed at portraying the 26 accomplices as bit players roped in, manipulated and intimidated by Fofana. They took no pleasure in tormenting Ilan and actually tried to alleviate the cruel punishment he imposed.

Michaël Doueib, an earlier Jewish victim of the Gang of Barbarians, is disgusted by their feigned innocence. "They didn't lift a hand to save him," he says. "An anonymous phone call, that's all they had to do." Lured to the same Bagneux neighborhood where Ilan would be jailed two weeks later, tied up and mercilessly beaten, Doueib escaped because residents who heard his screams called the police.

He claims police investigators rejected his offer of information, phone numbers, descriptions and other evidence that could have led them to the gang.

Whatever the verdict, we will be left with the troubling impression that the more this evil of Jew-hatred eats into the tissue of French society, the more it will be shrouded in artificial doubts and fabricated subtleties. This secret trial leaves Ilan Halimi once again illegally confined, isolated, bound and gagged, helpless to awaken dead hearts and warn potential victims.